1. Field of the Invention
The present invention relates to a method for selecting one wireless base station, with which a terminal is to communicate, from multiple base stations in a wireless system.
2. Description of the Related Art
Recently, wireless communication systems are becoming an infrastructure essential for the society. Wireless communication systems are classified roughly into two: one is a public network such as mobile phones and PHS where corporations provide facilities such as base stations and the other is a private network such as a wireless LAN, Bluetooth (registered trademark), and Zigbee (registered trademark)) where users prepare facilities such as base stations. Wireless communication devices for a private network, which are easy to install and low in cost, are being widely installed to allow users to communicate wirelessly wherever they are.
Wireless communication standards such as a wireless LAN and Bluetooth (registered trademark) were created with focus on data communication via personal computers and information communication terminals. Recently, however, there is a rapidly increasing need for various types of service other than data communication with the result that the wireless communication standard in a private network sometimes does not satisfy the communication quality required for those services. One typical example is the IP phone service. An IP phone is a service in which voice data is sent to the packet communication network, instead of the conventional switched telephone network, to implement the function equivalent to that available in the conventional telephone network. Wireless LAN standards such as IEEE802.11b are used to implement the IP phone function using a wireless terminal. Those standards, originally designed for data communication, involve problems such as a tendency to interrupt communication. A problem is generated also in handoff carried out for changing the connection destination from the current base station to another. In a wireless LAN, a terminal that connects to a base station before starting the communication selects a destination base station, for example, by selecting a base station from which a beacon (broadcast base signal) with the largest received power is received. Handoff is carried out in such a way that, if the received power of the signal received from the base station currently in communication falls below a predetermined threshold, the base station is changed to the next connection destination (finding a beacon with larger received power). At this time, if a low power state exists for a long time before the terminal is successfully connected to the next base station, the communication quality deteriorates. In addition, to solve the problem that the call is interrupted after the communication is once broken and before the next connection destination is found, a technology is developed for non-interrupted handoff. A service for wirelessly delivering a moving image in streaming mode also involves a problem that the moving image tends to be interrupted.
There is also a wireless-LAN-based position detection service disclosed in JP-A-2004-101254 (Patent Document 1). In this service, a wireless terminal must connect to a base station called a master base station. Although the master base station is installed in a good electric wave environment to ensure good position detection accuracy, the position detection accuracy is decreased if the terminal connects to a base station other than the master base station. Another problem is that, when hand off occurs, the terminal cannot connect to the next master base station.
As described above, a wireless communication method used in a private network has a problem that the method cannot satisfy the communication quality required for the wireless communication services that are diversified recently. Especially, though the installation positions of base stations are determined in a public network by calculating the electric wave range, such a calculation is not usually carried out in a private network. Instead, the installation positions of base stations in a private network are determined in many cases based on the ease of connection to a wired communication network or on the arrangement of furniture and fixtures. Therefore, a simple method, in which a wireless terminal connects to a base station from which the maximum received power is received, does not sometimes satisfy the quality required by services. For example, assume that, in a building where wireless base stations are arranged randomly, a wireless terminal has been connected to a base station on another floor from which a strong signal was received at a particular time. In this case, if the wireless terminal is moved even for a short distance, the quality of communication with that base station deteriorates greatly and communication with the base station is sometimes broken. To prevent such a problem from occurring, it is necessary to select a base station, with which a wireless terminal connects, according to the services that are used or according to the communication quality that is required.